Participants will be in this study for a minimum of 120 weeks and a maximum of approximately 4 years. In Step 1, patients are randomly selected to receive 1 of 3 blinded treatment regimens: abacavir (ABC)/lamivudine (3TC)/zidovudine (ZDV)/efavirenz (EFV), ABC/3TC/ZDV, or 3TC/ZDV/EFV. Patients with confirmed virologic failure on Step 1 and two successive plasma HIV RNA levels of 10,000 copies/ml or greater must register to Step 2. Patients with confirmed virologic failure on Step 1 and whose plasma HIV RNA is under 10,000 copies/ml may remain on Step 1 or register to Step 2. [AS PER AMENDMENT 04/11/03: Discontinuation of Arm B was recommended. Consequently, Arms A and C were unblinded to EFV but not to ABC. A number of options are available for patients originally randomized to Arm B.]